810 
ME.  HANCOCK  ON  THE  OEG-ANIZATION  OF  THE  BEACHIOPODA. 
of  these  organs  is  not  essential  to  the  performance  of  their  functions.  In  Atrypa,  for 
instance,  in  which  the  arms  were  rolled  exactly  as  in  Rhynchonella^  the  calcareous  sup- 
port prohibited  not  only  the  unrolling,  but  any  movements  whatever  of  the  coil.  It 
would  therefore  certainly  be  erroneous  to  conclude  that  in  the  latter  it  is  necessary  to 
the  due  exercise  of  their  function,  that  they  should  be  thrust  umnlled  out  of  the  shell. 
Such  an  opinion  can  only  be  maintained  on  the  ground  that  a special  apparatus  is  pro- 
vided for  the  purpose,  or  that  direct  observation  shows  it  to  be  correct.  We  have  seen, 
however,  in  this  form,  that  no  such  apparatus  is  really  in  existence ; and  nith  regard  to 
observation,  we  have  the  evidence  of  Mr.  Baerett^  who  had  frequent  opportunities  of 
examining  the  animal  alive,  but  never  saw  the  arms  in  the  least  degree  protruded. 
That  the  arms  are  instrumental  in  opening  the  valves  is  equally  improbable,  as  special 
means  are  provided  for  this  purpose  in  both  the  articulated  and  unarticulated  Bra- 
chiopods. 
In  Lingula  anaiina  the  brachial  organs^  are  strong  and  fleshy,  and  rise  from  the  back 
of  the  pallial  chamber  in  the  usual  manner.  They  are  entirely  without  calcareous 
support,  even  at  their  origin;  and  form  two  spiral  coils,  with  six  or  seven  turns 
each,  directed  inwards  and  upwards.  The  arms  are.  very  thick  at  the  base,  taper  more 
suddenly  than  in  the  articulated  species,  and  terminate  in  fine  points.  On  making  a 
transverse  section®  they  are  found  to  differ  in  organization  from  those  of  Rhynchonella,  to  ■ 
which,  externally,  they  bear  the  strongest  resemblance.  Instead  of  one,  as  in  that  genus, 
there  are  two  great  brachial  canals,  which  may  be  denominated  respectively  the  anterior 
and  the  posterior.  The  former  is  the  equivalent  of  the  great  canal  in  the  other  Bra- 
chiopods,  and,  like  it,  terminates  at  the  side  of  the  (esophagus  in  a blind  sac.  It  is  pretty 
regularly  cylindrical,  with  the  walls  excessively  thick,  being  composed,  for  the  most 
part,  of  a white  cartilage-like  substance,  which  is  most  developed  towards  the  side  oppo- 
site the  cirri.  These  latter  organs  are  supported  upon  a semi-cartilaginous  grooved 
ridge,  which  is  very  similar  to  that  in  Waldheimia^  and  in  like  manner  supplied  with  a 
very  ample  brachial  fold.  Muscular  fibres  are  also  provided  for  the  movements  of  these 
parts ; but  the  fibres,  though  very  similarly  arranged,  are  not  so  extensively  developed. 
This  canal  is  lined  with  a muscular  stratum,  the  fibres  of  which  run  in  a transverse 
direction  in  the  vicinity  of  the  brachial  fold ; elsewhere  they  are  longitudinal  or  slightly 
diagonal. 
The  posterior  canal  is  much  flattened,  and  stretches  along  the  inner  surface  of  the 
arm,  extending  from  the  base  of  the  cirri  half-way  round  it.  This  canal  has  the  appear- 
ance of  being  formed  by  a process  of  the  pallial  membrane  similar  to  that  which  extends 
along  the  lateral  portions  of  the  loop  in  Waldheimia.  The  posterior  canals  of  the  two 
arms  terminate  at  the  sides  of  the  oesophagus  behind  the  anterior  canals,  and  are  sepa- 
rated from  each  other  by  a considerable  space,  which  is  divided  on  the  median  hne  by  a 
delicate,  membranous  septum.  The  two  chambers  thus  formed  communicate  nith  the 
perivisceral  cavity,  by  two  small  oval  orifices*  placed  one  on  each  side  of  the  septum; 
' Op.  cit.  ^ Plate  LXIV.  fig.  3.  ' Plate  LXV.  figs.  7,  8.  ^ Plate  LXVI.  fig.  4. 
